1. Field of the Invention
The present invention relates generally to a method of mounting a window glass on a vehicle body, in which a robot is used for fitting the window glass to a window frame portion of the vehicle body which is conveyed to a window glass setting station provided in a vehicle body assembly line.
2. Description of the Prior Art
In a vehicle body assembly line, vehicle bodies having passed through a coating process are supported by respective carriers mounted on a vehicle body carrying conveyor and transported successively into a window glass setting station by the vehicle body carrying conveyor so as to be provided thereon with window glasses. At the window glass setting station, an industrial robot is installed for holding the window glass on which an adhesive agent has been put with a predetermined pattern in a pretreatment process and fitting the window glass automatically to a window frame portion of each of the vehicle bodies conveyed into the window glass setting station.
However, the vehicle body conveyed into the window glass setting station is usually accompanied with positional deviations arising in relation to the robot due to, for example, unavoidable errors in size of each vehicle body and each carrier supporting the vehicle body, errors in position of the vehicle body in relation to the carrier or errors in transportation of the vehicle body performed by the vehicle body carrying conveyor. Therefore, in the case where the robot holds the window glass which is to be fitted to the window frame portion of the vehicle body and operates to move along a predetermined working path in order to carry the window glass to the window frame portion of the vehicle body, it is feared that the window glass is not fitted properly to the window frame portion of the vehicle body by the robot so as not to be appropriately mounted on the vehicle body.
For the purpose of avoiding such a disadvantage, there has been proposed, by the same assignee as the present application, as disclosed in U.S. Pat. No. 4,715,772, a method of fitting a window glass to a window frame portion of a vehicle body including the steps of detecting a location of a positioning member, which is attached to the window frame portion of the vehicle body for defining a position of the window glass, by means of an image sensor such as a miniature camera employing a charge coupled device (CCD) in the window glass setting station, and modifying the working path along which the robot moves for carrying the window glass to the window frame portion of the vehicle body in response to a detection output of the image sensor so that the window glass carried by the robot is properly fitted to the window frame portion of the vehicle body.
In the event of such a method using the image sensor, however, since the location of the positioning member cannot be detected correctly by the image sensor when the window frame portion of the vehicle body is painted to be similar in brightness of color to the positioning member, it is still feared that the window glass carried by the robot is not fitted properly to the window frame portion of the vehicle body conveyed into the window glass setting station. For example, when the positioning member is colored white and the window frame portion of the vehicle body is painted white, light cream or the like so that there is only a small difference in brightness of color between the positioning member and the window frame portion of the vehicle body, the image sensor which has been ordinarily put to practical use is unable to distinguish clearly the positioning member from the window frame portion of the vehicle body surrounding the positioning member and produces an incorrect detection output concerning the location of the positioning member, and therefore the working path of the robot cannot be modified properly.